218 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 3, 



used to denote homologous parts of the teeth of the tichorhine and 

 megarhine species *. 



3. MilTc Dentition. — The Hyaena-den at Wokey Hole, which has 

 afforded a vast quantity of the remains of the tichorhine Ehinoceros, 

 has also yielded the best examples of the milk-teeth of the leptorhine 

 species, in two fragments of an upper and lower molar. Those in 

 the Museums of Oxford and the Geological Society of London, from 

 Kirkdale, are so badly preserved as to be unworthy of a detailed 

 notice. The fragment of the upper milk-molar, consisting of the 

 external lamina (fig. 1) of the second of the series occupying the 

 right upper jaw, is remarkable for the stoutness of the pyramidal 

 second costa (Jc 2), which stands out boldly above the plane of the 

 rest of the lamina, and is defined basally by two well-marked folds. 

 The third costa {Jc 3) , also pyramidal, but very faintly defined, occupies 

 the jposterior area (n) ; and between their apices is a small well-de- 

 fined elevation on the exterior of the crown-summit, which is pro- 

 bably a mere individual variation, as it occurs also in some of the 

 corresponding teeth of the tichorhine Ehinoceros. As compared 

 with the homologous tooth of the latter species, the leptorhine is 

 characterized by its smoothness, its small size, and the faint defini- 

 tion of its third costa ; as compared with that of the megarhine, by 

 the presence of the third costa, the stoutness of the second (Jc 2), but 

 especially by its small size. The average basal measurement of the 

 lamina of the second upper milk-molar is in the tichorhine 1*2 inch, 

 in the megarhine 1-35 to 1-53, in the leptorhine 0-9 inch. 



The second fragment (fig. 2), consisting of the unworn germ of 

 the third lower milk-molar, probably (from its condition) belonged to 

 the same individual as the preceding. Its external lamina (1) is 

 divided by a deep oblique groove (i) into two areas, of which 



* The following is the list of the terms and letters used to identify homolo- 

 gous parts in the most complex of the fossil Ehinoceros teeth (those of i?. ticho- 

 rhinus), and applicable to the teeth of all the species of the genus : — 



«= Anterior valley ="Vallis anterior," Brandt, = " Yallon oblique" in upper 

 molars, Cuvier. 



6=Posterior valley=" Yallis posterior," Brandt, = " Ecorchure au bord 

 posterieiir," Cuvier, — " Fossette posterieure," Blainville. 



c= Accessory valley ="Yallecula accessoria," Brandt. 



c?= Anterior collis = " CoUis anterior," Brandt, = " CoUine seconde," of upper 

 molars, Cuvier. 



e=Median collis="Collis medius," Brandt, = "La troisieme coUine" of 



ppe 



: Posterior collis = " Collis posterior," Brandt, = " Le bord post^rieur de la 

 dent," Cuvier, Blainville. 



^= Anterior combing-plate, a small process of enamel springing from the 

 external lamina and peculiar to i?. tichorhinus among the fossil species, 



A = Posterior combing-plate, a small process of enamel thrown forward into 

 the anterior valley. In the tichorhine species the imion of g and h cuts oif 

 c from h. 



2= Median groove on the external lamina. 



^=Cost£e = "costae," Brandt, on the external lamina. 



?=External lamina=" Collis externus," Brandt, = " CoUine premiere qui 

 suit exactement le bord," Cuvier, 



w = Anterior area, 



n = Posterior area. 



